In fuel cells operating at dead-end mode there is accumulation of both inert gas and water on the anode side of the cell. Water is accumulated in the channels and it is periodically removed by so called “hydrogen purge”, by opening the exit valve of the anode side for a short period of time.
The hydrogen purge has a dual function. It removes both accumulated inert gas as well as water from anode channels. The removal of inert gas can be directly measured using a hydrogen concentration sensor. However, there is no practical and accurate method for measurement for the removal of water, especially when the pressure drop over the anode is small. If the pressure drop is high and if the flow rate of re-circulated anode gas is known, then the change in pressure drop can be used for the detection of water removal.
When the purge duration and frequency can be optimized, fuel efficiency can be increased slightly (up 1-3‰).
The water accumulation on the anode side can be detected from the change in the differential pressure signal before and after the purge. However, the use of differential pressure is only possible if the flow rate on the anode gas remains constant and if the pressure difference is large enough. This is not the case, when the fuel cell is operated at low power or when the anode pressure drop is otherwise very small (under 10 mbar).
There is a need for a method for determination of water accumulation inside PEFC stack anode for example in in order to be able to develop an optimal purge strategy.
Pressure drop with a certain flow rate and gas composition is currently a primary diagnostic method for accumulation of water on the anode side. Recirculation flow rate can be easily calculated from the current (hydrogen consumption). When recirculation rate is low (pump used, instead of ejector) the pressure drop is low and use of pressure signal is more difficult. In addition, if diaphragm type of pump is used, the pulsing flow makes the use of pressure signal more difficult to measure.
It is at low flow rates, when also the water accumulation is also most severe, as cathode humidity is highest and flow velocity on the anode lowest. Low flow rates are also encountered during start-up and shut-down, when it is especially wet conditions in the cells. Therefore, at low flow rates an additional method for the detection of water accumulation in the channels is needed
Thus, there is a need for determining water accumulation at low load levels, this being difficult with traditional measurements (pressure drop dP between inlet and outlet).
To mention further problems relating to PEFC systems, hydrogen, which is consumed in hydrogen fuel cell system, contains always some amount of inert gases including also carbon dioxide. These gases enrich in fuel cell system, as hydrogen fuel cell system is operated in dead end mode with recirculation and purge.
Further, in fuel cells operating at dead-end mode the real fuel utilization is very difficult to measure on-line or during annual maintenance. Fuel utilization is, however, a key parameter to measure the efficiency and aging of the system. The fuel utilization will change during the life-time of the stack/system as there will be changes in stack component properties (membrane, gas diffusion layer, bipolar plate).
In long measurement, pressure and temperature of gas storage can be used for the calculation of fuel utilization. This requires highly accurate measurement of both pressure and temperature of the gas storage. On the other hand, momentary fuel utilization in certain operation point requires accurate measurement of hydrogen flow during a hydrogen purge. The measurement of gas flow rates during the purge is possible to perform in laboratory experiments (Nikiforow et al. 2012). However, in commercial fuel cell systems the required instrumentation is far too expensive.
Fuel efficiency is not accurately measured on-line in existing fuel cell systems. For this reason, determination of PEFC system aging, to mention one example, is difficult.
Thus, there is a need for methods making it possible to accurately measure of fuel utilization with reasonable instrumentation and cost.